Adam Clarke on Deuteronomy 22:30

A man shall not take his father's wife - This is to be understood as referring to the case of a stepmother. A man in his old age may have married a young wife, and on his dying, his son by a former wife may desire to espouse her: this the law prohibits. It was probably on pretense of having broken this law, that Solomon put his brother Adonijah to death, because he had desired to have his father's concubine to wife, 1Kgs 2:13-25.